In electron spin resonance spectroscopy, a microwave is applied to a sample placed in a static magnetic field that is swept. An ESR (electron spin resonance) spectrum is obtained by taking the first-order derivative of a microwave absorption curve as a function of the magnetic field. Thus, the absorption intensity is plotted on the vertical axis of the spectral chart and the magnetic field strength on the horizontal axis. Generally, the magnetic field strength (resonating magnetic field) of the resonant spectrum is given by ##EQU1## where .nu. is the frequency of microwaves, h is the Planck's constant, .beta. is the Bohr magneton, and the value of g is intrinsic to the material. The factor g is one of the most important factors, as well as the hyperfine coupling constant and the line width, in determining from what paramagnetic species the ESR spectrum arises. An unknown sample is identified as follows. A known marker is used so that an absorption line of the marker may appear in an ESR spectrum together with the absorption line of the unknown sample. The magnetic field strength is found from the g value of the absorption line of the marker. The g value of the unknown sample is calculated from the resonating magnetic field strength H of the unknown sample and from the microwave frequency .nu..
DPPH (2.0036), TCNQ-Li salt (2.0026), and Cr.sup.3+ (1.98) are used for calculating g values. Also, Mn.sup.2+ marker, or manganese marker, has been frequently used either in ESR measurement for quantitative analysis of a paramagnetic substance contained in an unknown sample or as an external reference for measuring the g value.
Since like substances have like g values, it is required to determine g values accurately to identify such substances accurately. Except for Mn.sup.2+ marker, however, every marker produces a single absorption line and so the magnetic field of a spectrum can be calibrated only at one point. Therefore, it is inevitable that g values are calculated at low accuracy.
On the other hand, Mn.sup.2+ marker gives rise to 6 absorption lines because of its nuclear spin quantum number I=5/2. If the g values of these absorption lines are known, therefore, the horizontal axis of the spectrum, or the magnetic field, can be calibrated at plural points. For this reason, the magnetic field strength of the absorption line of an unknown sample can be accurately calibrated. Hence, the g value can be accurately determined.
However, the g values of the individual absorption lines vary, depending on the frequency of the applied microwaves, because the six absorption lines of the Mn.sup.2+ marker are produced by its hyperfine structure. The g values of the third and the fourth absorption lines change relatively little, depending on the applied microwave frequency. In the past, it has been assumed that the g values of these two intermediate absorption lines are kept constant, irrespective of the applied microwave frequency, and the g values of the absorption lines of unknown samples would have been calculated from the g values of these two absorption lines.
In this method, however, high accuracy is not obtained in essence, because it neglects the fact that g values vary according to the applied microwave frequency.